This invention relates generally to filled fluoropolymeric composites. More particularly, this invention relates to a fluoropolymeric composite for use as an electrical substrate material which exhibits both a high dielectric constant (K') and a low thermal coefficient of dielectric constant (TCK'). This electrical substrate material is particularly useful as a laminate for manufacturing microwave circuits.
The electrical performance of electrical circuits and devices is highly dependant on the dielectric constant, K', of the dielectric medium. Thus, when the dielectric constant of a material changes with temperature, the electrical performance of the device will change as well.
A basic treatment of the factors affecting the temperature coefficient of dielectric constant (TCK') of homogeneous compounds is set forth in Equation 1 below: EQU TCK.sup.1 =K.sup.1 /3{(1/.alpha.)d.alpha./dT-3.alpha..sub.L }(1)
where .alpha. is the polarizability of the medium, d.alpha./dT is the change in the polarizability with temperature and .alpha..sub.L is the average linear coefficient of thermal expansion (CTE) of the medium. See A. J. Moulson & J. M. Hebert; Electroceramics. P. 223, Chapman and Hall, London, (1990).
This equation demonstrates that there are two basic factors affecting TCK'. The {(K'/3) (1/.alpha.) d.alpha./dT} term describes the change in K' with temperature due to the change in polarizability of the electrons in the medium. In principal, d.alpha./dT can be a positive, negative or null quantity. A priori prediction of d.alpha./dT based on molecular structure is not presently possible.
The second term of equation 1, -K' .alpha..sub.L, is simply the product of the dielectric constant and the average linear coefficient of thermal expansion. This term accounts for the change in the number of polarizable sites per unit volume due to the increase in volume with increasing temperature. With most materials over any reasonably wide temperature range, .alpha..sub.L is a positive quantity so the term, -K' .alpha..sub.L is generally negative.
A review of TCK' data by Moulson and Herbert demonstrate that for many common ceramic dielectrics the {(K'/3) (1/.alpha.) d.alpha./dT} is small compared to the -K' .alpha..sub.L term. Thus, the TCK' of many materials is a function primarily of the magnitude of the dielectric constant and CTE and TCK' is most often less than zero. This analysis also demonstrates why high K' materials usually possess a high TCK'.
This analysis also indicates that in order to achieve a low TCK' in a high K' compound, the -K' .alpha..sub.L term must be "compensated" by a positive d.alpha./dT. Since the Second World War, a segment of ceramic materials research has been devoted to identifying "temperature compensated" high K' dielectric materials. When referring to ceramic materials, " high K'" generally denotes K' values of greater than 30. See E. N. Bunting, Shelton, G. R., Creamer, A. S.; "Properties of Barium-Strontium Titanate Dielectric," J. Amer. Ceram. Soc., vol 30, n.4, pp 114-125, (1947). These efforts have resulted in a number of temperature stable ceramic dielectric materials with K' values in the range of 35-100. The most temperature stable of these materials are specified with a TCK' of 0.+-.30 ppm/.degree. C. and are classified by the EIA as "COG" class ceramic dielectrics. An alternative designation in the capacitor industry for materials which such a TCK' is "NPO." For the most part, these materials are used in the manufacture of "Class 1" capacitors for lower frequency (less than 100 MHz) electronic applications.
A number of such materials are commercially available. The Transelco Division of Ferro Corporation offers a ceramic composition of formula ReBaPbBiTiO.sub.3 under the tradename of "900-NH (K-90 NPO)." When tested at low frequencies, this composition exhibits a K' of 83 and DF of 0.00002 and meets the "NPO" specification for TCK'. According to the manufacturer, no high frequency (greater than 700 MHz) test data were available. Other such materials include "2M101.3 NPO" dielectric ceramic from Radio Materials Corporation of Attica, Ind., comprised of a neodymium and zinc oxide doped barium titanate. This material is also referred to as "N60". As with the Ferro material, this ceramic dielectric was intended for capacitor manufacture and no high frequency test data were available. Another material of this type is "COG-100," a proprietary ceramic capacitor dielectric manufactured by Dimat, Incorporated of Cedarburg, Wisc. As with the previous examples, this ceramic dielectric was intended for capacitor manufacture and no high frequency test data were available.
Some temperature-compensated dielectric ceramics have been identified for high-frequency electronic applications. O'Bryan et al specifically identified a ceramic composition, Ba.sub.2 Ti.sub.9 O.sub.20, with a K' of 39.8, a TCK' of -24 ppm/.degree. C. and a low dielectric loss at microwave (4 GHz) frequencies. See H. M. O'Bryan, Thomson, J., Plourde J. K.; "A new BaO-TiO.sub.2 Compound with Temperature-Stable High Permittivity and Low Microwave Loss," Journ. Amer. Ceramic Soc., Vol. 57, No. 10, pp. 450-452, (1974). This material, commonly referred to a barium nanotitanate (BNT), is commercially available as fired shapes from Trans Tech, Inc. (Division of Alpha Industries) for use in high frequency electrical systems. Trans Tech, Inc. also commercially offers a spherical spray-dried BNT powder. BNT is also available as a fine ceramic powder (for Class 1 capacitor manufacture) from Dimat, Inc. Trans Tech also offers a higher K' ceramic as fired shapes, "material type 9000," a barium samarium titanate. Material type 9000 exhibits a K' of 90.5 and a TCK' of approximately 12 ppm/.degree. C. at a frequency of 3 GHz.
In short, a number of "NPO" ceramic compounds are commercially available. Most have been formulated for and are used in the manufacture of ceramic capacitors that are used in comparatively low frequency electronic systems. Those few NPO ceramic compounds that have been formulated and tested for high frequency (&gt;1 GHz) electronic systems are sold as fired shaped and are not broadly applicable for use as fillers in composite materials systems.
Many polymeric composite materials are presently available for use as a laminate for microwave frequency electronic applications. Prevalent amongst these materials are composite systems based on PTFE (poly(tetrafluoroethylene)) and other fluoropolymers (such as FEP (poly(tetrafluoroethylene-co-hexafluoropropylene) and DuPont's PFA). Fluoropolymeric composites are desirable due to their excellent high-frequency electrical properties and excellent high temperature and solvent resistance. When referring to organic polymer based composite circuit substrate, low K' generally denotes a K' value of less than 3.0, while high K' implies a value of greater than about 4.0.
A common class of fluoropolymeric composite microwave laminates are those that are reinforced in the XY plane with either woven glass cloth or random glass microfiber. Examples of such materials are Rogers Corporation's RT/duroid.RTM. 5880 and Ultralain.RTM. 2000 and the material described in U.S. Pat. 4,886,699, assigned to the assignee hereof and incorporated herein by reference. The dielectric constant values of these types of materials commonly ranges from 2.17 to about 2.65. This class of materials exhibits comparatively high Z-axis coefficients of thermal expansion (CTE), ranging from +125 to +250 ppm/.degree. C. In spite of the high Z-axis CTE of these materials, the thermal coefficient of K' is relatively low. The TCK' of RT/duroid 5880 has been measured to be approximately -75 ppm/.degree. C. at a frequency of 10 GHz over the temperature range of 20.degree. C. to 250.degree. C. This comparatively good TCK' is due in part to the relatively low K' of this class of materials.
Another type of fluoropolymer composite useful for microwave laminates is described in U.S. Pat. No. 4,849,284, assigned to the assignee hereof and incorporated herein by reference. A preferred embodiment of this invention is sold by Rogers Corporation to the microwave circuit board industry under the trademark RT/duroid.RTM. 6002. This composite material consists of fused amorphous silica, PTFE and E-glass microfibers. It exhibits a K' of 2.94, a Z-axis CTE of about 24 ppm/.degree. C. and a TCK' of about +20 ppm/.degree. C. The small value for the TCK' allows for stable electrical performance of circuits made on RT/duroid 6002 over a wide range of temperature. This feature is highly valued by designers of microwave circuits.
Other commonly assigned patents and patent applications describing fluoropolymer composite materials of this type include U.S. Pat. Nos. 5,024,871; 5,061,548; 5,077,115; 5,149,590; 5,194,326 and 5,198,295 and U.S. patent application Ser. Nos. 07/641,427; 07/703,633; 07/704,983; 07/705,624; 07/705,625; 07/808,206 and 07/897,244. Other patents describing materials of this type include U.S. Pat. Nos. 4,985,296 and 5,055,342.
While the aforementioned fluoropolymeric composite materials generally exhibit low dielectric constant, there is also a need for comparatively high K' (K'.gtoreq.4) fluoropolymeric composite materials such as those described in U.S. Pat. No. 4,335,180, (which is assigned to the assignee hereof and incorporated herein by reference) as well as U.S. Pat. No. 4,996,097. High K' materials of this type are sold by Rogers Corporation under the trademark RT/duroid 6006 (K'=6.15) and RT/duroid 6010 (K' of 10.2 to 10.8). This class of materials is generally made by adding titania ceramic filler to increase the K' of the material. These high K' materials exhibit z-axis CTE's of about +45 ppm/.degree. C. to +80 ppm/.degree. C. and TCK' values of about -500 ppm/.degree. C. to -600 ppm/.degree. C. The major factor in causing the high TCK's of this class of materials is the high TCK' of the ceramic filler that is added to modify the K' (although as is demonstrated in the examples below, it has been discovered by the inventor herein that the CTE also plays a major role). Titania (TiO2) itself exhibits a TCK' of approximately -750 ppm/.degree. C.
Presently, applicants are not aware of any fluoropolymeric based composite for use as an electrical substrate material which exhibits both high K' and low TCK'. However, applicants are aware of other non-fluoropolymeric based composites useful as microwave dielectric materials which have temperature stable high dielectric constant properties. Most notable for this characteristic is the polybutadiene based composite described in commonly assigned U.S. Pat. No. 5,223,568 (all of the contents of which are incorporated herein by reference) and sold by Rogers Corporation under the trademark TMM.RTM.. Such TMM.RTM. microwave substrate comprise a thermoset matrix (predominately poly (1,2-butadiene) liquid resin that is highly crosslinked) and a ceramic filler blend of silica and barium nanotitanate (BNT). The BNT powder exhibits a mean particle size of approximately 2.5-4.0.mu.. Because of the nature of the TMM.RTM. laminate mixing process and liquid resin system, unlike PTFE composites, it exhibits no porosity at filler loadings of up 65 volume %, even with this small particle size distribution filler.
Unfortunately, it has been determined that merely adding such BNT filler (or other similar class 1 capacitor ceramic powders) into PTFE (as in the TMM.RTM. polybutadiene based laminates) composites leads to unacceptably high porosity and therefore exhibits unacceptable uptake of organic solvents and low surface tension surfactant treated aqueous solutions used during circuit board manufacture. The uptake of such solvents and solutions can lead to processing difficulties, long term reliability problems and increased dielectric loss if they are not fully removed before further processing. While TMM.RTM. laminates do exhibit the important high K' and low TCK', fluoropolymer based laminates exhibiting such properties are nevertheless desired because, in general, a fluoropolymer (e.g., PTFE) composite will exhibit lower loss, higher peel strength and superior high temperature resistance relative to polybutadiene based composites when used as an electrical substrate material.